A portable computer such as a notebook-size personal computer (hereinafter referred to as a notebook PC) or a PDA (personal digital assistant) is usually provided with a communicating function with respect to a wireless network, such as a wireless LAN (local area network) module. Moreover, it is demanded that the provided wireless LAN module should cope with an increasingly high-speed communication standard. Heretofore, wireless communication having a transmission speed of 54 Mbps (Megabits per second) at maximum in a physical layer has been put to practical use with the aid of IEEE802.11a/b/g, but especially, an FTTH (fiber to home) having a transmission speed of 100 Mbps or more has spread even to general households, and hence it is demanded that the transmission speed of wireless LAN should also be in excess of 100 Mbps. Therefore, as the wireless LAN having a transmission speed of 100 Mbps or more in an MAC (media access control layer) layer, IEEE802.11n is to be standardized and put to practical use in 2007.
As a central technology which realizes an increase of the transmission speed of this wireless LAN, a space division multiplexing (SDM) transmission technology using a multiple input multiple output (MIMO) is employed in IEEE802.11n. In this technology, a plurality of antennas are installed on transmission and reception sides, respectively, and transmission data is divided by the number of the antennas, modulated at an equal frequency, and simultaneously transmitted in parallel. In consequence, a capacity of the simultaneously transmittable data can be increased. However, signals transmitted from the respective transmission-side antennas are influenced by reflection and decay of a transmission path, and hence signals received from the respective reception-side antennas are strained and these strained signals are synthesized. Therefore, the strain of the received signals needs to be estimated to restore the transmission signals. FIG. 11 is a conceptual diagram of the SDM technology using the MIMO. A transmission station 601 includes M antennas, and a reception station 603 includes N antennas, respectively. The transmission station 601 divides input information into M information channels by serial-parallel conversion, and transmits these pieces of the information from the respective antennas. The pieces of the information transmitted from the M antennas of the transmission station 601 reach the N antennas of the reception station 603, respectively. Between the stations, M×N MIMO channels or propagation paths referred to as a multipath are present. Furthermore, they include strain components due to reflection, decay and the like, respectively. Assuming that the signals transmitted from the respective antennas of the transmission station 601 are t1 to tm (transmission signal vectors) and the signals received by the antennas of the reception station 603 are r1 to m (reception signal vectors) in an equation 605, a transmission function can be obtained from a pilot signal and a preamble signal to estimate the transmission signals from the received signals.
On the other hand, an orthogonal frequency division multiplexing (OFDM) is a multicarrier communication system which multiplexes signals with a plurality of sub-carriers, and a sub-carrier interval is regarded as an inverse number of a signal period so that the sub-carriers are independently separable. FIG. 12 is a conceptual diagram of an OFDM technology. In the OFDM, a guard zone (GT) 613 obtained by copying a part of a symbol 611 is added before each symbol. On a reception side, a signal in which a direct coming wave 615 is mixed with a delay wave 617 due to the reflection or the like is received. However, owing to the presence of the GT 613, a demodulation zone of the symbol 611 included in the direct coming wave 615 is not influenced by the symbols before and after the symbol 611 included in the delay wave 617. In consequence, the influence of the delay wave can be reduced to perform demodulation. Furthermore, since the influence of the delay wave is reduced, an equation to estimate the transmission signal from the received signal according to the MIMO can be simplified. Therefore, the OFDM has high affinity with the MIMO, and a MIMO-OFDM in which both of the MIMO and the OFDM are combined is expected as a nucleus of a mobile body communication technology of the next generation.
As a technology in which a wireless terminal provided with a plurality of antennas using the MIMO and receiving sections corresponding to the respective antennas selects the receiving section to be used, Masahiro et al. (Japanese Unexamined Patent Publication No. 2006-115414) (hereinafter “Masahiro”) teaches a technology in which a frame sent from a base station is analyzed to select the receiving section. As a technology in which the antenna to be used is simply selected from the plurality of antennas, Satoru et al. (Japanese Unexamined Patent Publication No. 2004-260338) (hereinafter “Satoru”) teaches a technology to select a reception system having a large received signal level. Kohiya et al., Wireless Broadband Textbook, Tokyo Denki University Publication Bureau (2006) and Morikura et al. (editors), Revised Version 802.11 High-Speed Wireless LAN Textbook, Impress (2005) describe the basics of MIMO and OFDM technologies. Deguchi et al., Key Technology of Portable Terminal in Next-Generation Mobile Communication System, Toshiba Review Vol. 60, No. 9, Toshiba Corp. (2005) describes a technology to reduce a processing amount in a MIMO-OFDM system.
While the above systems and methods allow for wireless communications, a need has arisen for increasing the ease and flexibility of employing such wireless communications as well as addressing other shortcomings of the above described systems and methods.